Waveform light detection and ranging systems (LIDARs) are increasingly used to measure features of the environment from airborne or on-orbit platforms. Waveform LIDARs are a powerful analytical technique to profile three-dimensional diffuse targets. From above, they can be used to measure the carbon content of forests, profile the density of organisms in the ocean, and characterize atmospheric phenomena. An orbiting observatory is an ideal platform from which to monitor these effects. Unfortunately, imaging LIDARs, even in low Earth orbits, are limited by the total optical power returned from the distant Earth. This in turn limits the number of available pixels over which to spread the available returned light. While there is insufficient light to illuminate millions of pixels in a large two-dimensional array, there may be sufficient light for tens, hundreds, or even thousands of pixels in a two-dimensional or in a line array. This orbiting situation naturally lends itself to a pushbroom LIDAR system with up to about a thousand pixels arranged as a line array in the cross track direction. This is in contrast to an airborne LIDAR platform where distances may be reduced from 400 km (Low Earth Orbit) to 13 km (airplane cruising levels), resulting in significantly more light: (2×400 km)^2/(2×13 km)^2, or almost 1000 times more light.
The returns from a target typically arrive at the LIDAR system at nearly the same time. As a result, and in particular for an imaging waveform LIDAR system, a large number of high speed storage elements are required. These storage elements may be in the form of analog or digital storage elements. If digital storage elements are utilized, a large number of high speed digitization elements are also required. In particular, one such analog storage element or one such digitization element/digital storage element pair is required for each pixel of the detector. In addition, the digitization and storage elements must be capable of operating at high speeds. For example, for a LIDAR system providing one meter resolution, data rates are above 160 MHz, with 400 MHz being more typical. Operating at these speeds, an analog to digital converter requires on the order of one watt of power. If 1,000 analog to digital converters are needed, the power requirement for those converters is 1,000 W, which is too high for deploying as part of an on-orbit platform.
LIDAR receivers that dissipate very low power provide a major advantage and have been in existence for a number of years. For example, systems have been developed that utilize one high speed low power analog memory per signal line. However, the resolution in the third dimension (distance) of the three-dimensional imaging of such waveform LIDAR systems has been limited. In particular, such systems have utilized analog and digital circuitry including analog memory contained in a ROIC pixel directly below and within an area defined by a detector pixel. As a result, there is insufficient area in which to provide sufficient analog memory to store a high resolution time history of the returned signal. This lack of memory limits the resolution of the time return of the LIDAR signal. For example, only 20 or 44 analog samples can be captured for each pixel per pulse. Applications in true volumetric imaging of diffuse targets require substantially more than 44 time samples.